OP I have experience in so far as I have been on the other side. Not what you are asking I know but here are my thoughts.
My mum changed her will after my dad died to leave her estate to me and exclude my siblings. She did this for reasons such as I cared for her while she was sick/end of life but also because she had always been much closer to me than my siblings. My siblings now don't speak to me because of 'the money' so they clearly blame me for what happened. They did not contest the will because my mum changed hers using a solicitor. She also included a letter to be sent to my siblings to explain the changes to the will. She also told them before she died (upon which time my siblings stopped talking to me).
My mum did all this of her own free will. You can disagree with her decision but it was her decision. Luckily for me my mum had a scan for dementia not long before she died and she didn't have it. We did this because she started saying lots of weird stuff towards the end (this was about 3 years after she changed her will). The police also spoke to my mum as I think her bank had alerted them to the fact she was paying me money to care for her and also gifting me money to buy things. The police had a good chat with her (when I wasn't there obviously) and said she was very 'sharp'.
She looked very frail but still knew her own mind at that time.
Despite all this evidence that my mum acted of free will (hiring a solicitor, speaking to the police, including a letter with the will, telling my siblings) my siblings are not talking to me. I think the point I am trying to make is in their case I believe they are so hurt by my mums actions that they have displaced their anger onto me. I mean why else would they not be speaking to me when there is so much evidence that my mum changed her will of her own free accord.
Emotions run very high at times like this and I suppose what I am trying to say is I have siblings who now 'blame' me and don't talk to me for something I had no control over. I am guessing it is too difficult to blame a dead person or to face up to being cut out of the will (which is of course hurtful) so they are displacing their anger onto me.
It does not matter in so far as we were not close and I never heard from them much anyway (I usually don't even remember that they are not talking to me).
Ironically when my mum told me what she had done I actually said to her several times 'are you sure you want to send them a letter and change the will because that will be the last thing you ever say to them' and she was adament she did. She was angry because they refused to help look after her so she was punishing them I guess.
I suppose what am saying is you are very understandly upset just now and grieving the loss of your mum. If she has changed her will it will be very hurtful and it is natural to try to avoid the pain of this rejection by your mum by blaming someone else. If you have evidence (in writing) or proof of evidence that your mum was co-erced then of course by all means challenge the will if it has been changed. From your post you sound emotional, angry and perhaps not thinking as clear as you could (which I totally understand).
My advice would be
Check the will to see if it has been changed. Then see if you have any evidence of wrong doing. Then get advice from a very reputable lawyer. What people have said is correct. It will be stressful and the lawyer fees will eat up a ton of it. Check to see if she did an letter to explain the changes to the will.
The saddest thing about my story is my siblings decided I was somehow to blame and stopped speaking to me. They never gave me a chance to decide what I would do when the estate was settled. In other words I might have offered some of it to them. It is not very nice to be accused of robbing your own mother when no such thing occurred. Unfortunately I don't think even offering money to my siblings would have fixed things as it would not have stopped them knowing their own mother cut them out of her will and that is what they are angry /hurt about.
If I was close to my siblings before all this I would be very upset with being cut off. As it is it makes literally no difference to me as I very rarely heard from them. Are you close to your sister. Is the relationship worth losing?
I'm very sorry for your loss and for all your anger and upset. I hope you manage to resolve it to your satisfaction.
I had POA for my mother although it was set up at the same time as she changed her will (I was not allowed in the room or building when she was changing her will as the solicitors wanted to have a big chat with her and check her competency I guess without me there).
I hope my post does not make your more upset.